In general, battery chargers provide the rudimentary function of converting a readily available AC voltage into either a varying or stead state DC voltage for charging a rechargeable battery. Beyond this general definition, battery charging techniques diverge in a host of different directions to provide recharging facilities for batteries. For example, many conventionally available battery chargers are pluggable into a 60 Hz, 110 VAC outlet for providing either a half-wave or full-wave rectified signal for charging, for example, a 12-volt automobile battery. Such chargers are generally not universal, in that they can only be plugged into the 110 VAC source and are capable only of charging a 12 volt battery. Similarly, other commercially available chargers, such as for 1.5 volt rechargeable batteries, are usable only in conjunction with the 1.5 volt batteries. While such type of chargers may or may not be electrically capable for use with other types of AC sources, such as the European 50 Hz, 220 VAC outlets, the AC cords permanently attached to such battery chargers cannot be used with the receptacles of the European AC or other systems. Because of the diverse types of AC distribution systems throughout the world, the foregoing conditions have required that manufacturers develop battery charging systems that satisfy the technical requirements of the different types of batteries, as well as the availability for use in countries having line power ranging from 60 Hz, 110 VAC to 50 Hz, 260 VAC. Often, in order to accommodate the different AC power systems in the world, manufacturers provide different models of battery chargers for use with the different AC distribution systems. This not only increases the manufacturing costs, but also requires additional inventory space for the full line of battery chargers.
A common practice in the design of battery chargers is to utilize a transformer and a rectifier to produce a unipolar voltage, and a series resistance to limit the charging current provided to the battery. Not only is the series resistance wasteful of power dissipated as heat, but also such type of chargers are often unable to eliminate overcharging of a battery, which result can be detrimental to sealed lead-acid type of batteries. In other words, if a battery charger continues to provide current into an otherwise fully-charged lead-acid battery, uneven charging of the battery plates can develop, thereby adversely affecting the life thereof. Other complex battery charging systems provide a very stable DC supply voltage with current sensing and control elements as well as circuits to limit battery charging time or rate. Such type of battery chargers can be very expensive to manufacture.
From the foregoing, it can be seen that a need exists for a universal battery charger that can be readily adapted for use with a variety of different AC distribution systems, and in which the AC conversion circuits are simplified and very cost effective. Another need exists for an economical battery charger which provides a charging current only when needed, and which provides charging energy having parameters that are independent of the input AC voltage to which the charger is connected. A further need exists for a battery charger which will not produce an output voltage unless a battery is connected thereto, and then only to the extent necessary to fully charge the battery and maintain it charged without overcharging.